The primary account of Aaron's death appears in Bamidbar 20:23, where God speaks to Moses and Aaron at Mount Hor on the boundary of Edom, and the narrative concludes in Bamidbar 20:22–29, where Moses strips Aaron of his priestly vestments, transfers them to Eleazar, and Aaron dies on the summit — after which all of Israel mourned him for thirty days.
A second mention appears in Devarim 10:6, which states that Aaron died and was buried at Moserah, and that his son Eleazar became priest in his stead — a passage that Rashi (Devarim 10:6) and the Ramban (Devarim 10:6) both note is textually difficult, since the itinerary in Bamidbar places Moserah eight stations away from Mount Hor, not identical to it.
A third reference comes in Devarim 32:50, where God instructs Moses that he will die on the mountain he is ascending and be gathered to his kin, explicitly invoking the precedent that 'your brother Aaron died on Mount Hor and was gathered to his kin.'
Rashi (Bamidbar 20:22–29) adds that Mount Hor was shaped like a small apple atop a large apple, and that three mountains were left standing — Sinai for the giving of the Torah, Mount Hor for Aaron's burial, and Mount Nebo for Moses' burial.